This invention relates to raster distortion correction circuitry and more particularly, to top and bottom pincushion distortion correction circuitry.
Ordinarily, television receivers have a cathode ray tube with a deflection yoke affixed thereto. Horizontal and vertical deflection circuitry in conjunction with the deflection yoke provide a varying electromagnetic field whereby the electron beam of the cathode ray tube is deflected to provide a raster. This raster is preferably rectangular in shape but, unfortunately, various types of electron beam distortion cause the raster to deviate from the desired rectangular configuration.
A well-known form of raster distortion is known as pincushion distortion, and more particularly, top and bottom pin-cushion distortion. Herein, the horizontal scan lines forming the raster tend to have a maximum deviation or bowing intermediate or central of the ends of a horizontal scan line. Also, the deviation of bowing varies from a minimum at the vertical center of the raster to a maximum at the top and bottom of the raster. Moreover, the distortion or bowing of the scan lines is downwardly from the top of the raster and upwardly from the bottom toward the center of the raster.
Known arrangements for correcting the above-described top and bottom pincushion distortion includes alteration of a current varying at the vertical frequency fv by a current varying at the horizontal frequency fh. This alteration of the vertical current fv by the horizontal current fh must occur at the phase and magnitude such that the undesired distortion or bowing of the horizontal scan lines, is in effect, cancelled.
One known form of apparatus for effecting correction of the above-described top and bottom pincushion distortion is disclosed in U.S. Pat. No. 3,463,961 issued to C. B. Neal et al. on Aug. 26, 1969. Therein, a pair of amplifying devices are connected in push-pull with a signal at field scanning frequency fv and opposite phases applied to the amplifiers. Also, a signal at the horizontal scan frequency fh is coupled to each of the amplifiers. The output from the amplifiers is coupled to the vertical deflection winding of a yoke associated with a cathode ray tube wherein the desired pincushion correction is effected.
Another known form of apparatus for effecting top and bottom pincushion correction utilizes a saturable reactor whereby an appropriately modulated horizontal frequency component is introduced into the vertical scanning path. Factually, a wave component varying at both horizontal and vertical deflection frequencies is produced and added to a vertical deflection wave for impression upon the vertical windings of a deflection yoke. In this manner, the top and bottom deviations in the horizontal scan lines are corrected and a substantially rectangular-shaped raster is effected.
Although the above-mentioned techniques have been employed in numerous television receiver designs with a great deal of success, it has been found that such techniques do leave something to be desired. For example, multiple amplifying devices are expensive and subject to deterioration and replacement. Saturable reactors are also expensive, subject to thermal runaway and failure, and tend to radiate considerable stray and undesired electric and magnetic fields. Moreover, saturable reactors are difficult to design and adopt to specific chassis and even more difficult to employ where independent and differing amounts of top and bottom correction is desired.